BiOrb Air Stone Replacement, THREE Pack. Suitable for ALL BiOrb Classic, Tube, Flow, Halo and Life Aquariums. The BiOrb air stone, located at the base of the aquarium, breaks up the flow of air into the aquarium into a mass of bubbles. After a time the air stone can become blocked by calcification, especially in hard water areas, reducing the flow of air into the aquarium. Replacing the air stone every few months will keep your aquarium bubbling away nicely. In some hard water areas it may be necessary to replace the air stone more frequently, but once every 6 months should be sufficient for most owners.